The present invention relates to a twin-shaft vacuum pump including a pump chamber, a pair of rotary pistons disposed in the pump chamber and end plates laterally delimiting the pump chamber as well as a housing ring which peripherally delimits the pump chamber.
Twin-shaft pumps include, for example, Roots pumps whose rotary pistons have an approximately figure-eight shaped cross section, Northey pumps with claw-shaped rotors, screw pumps and the like. The pairs of rotary pumps rotate without contact relative to one another and to the pump chamber walls and cause the pumping medium to be conveyed from the inlet to the outlet of the pump. These twin-shaft pumps are particularly suitable for use as vacuum pumps since no sealing and cooling means are required in the pump chamber and there thus exists no danger of contamination from the sealing agent.
Due to the contact-free arrangement of the rotary piston in the housing, it is unavoidable that some of the conveyed medium flows back. The volumetric efficiency of twin-shaft pumps of this type is therefore defined by the ratio of the effective quantity of gas conveyed to the theoretically conveyable quantity of gas. The less play there is between the individual rotary pistons and between the rotary pistons and the pump chamber walls, the less return flow occurs, that is, the better is the volumetric efficiency of the pump. However, the selection of any desired small amount of play is not possible for thermal reasons. During operation, the pump heats up. The possible range of existing plays is therefore reduced so that there exists the danger of the rotary pistons scraping against the housing. If the rotational speed is increased, as is desirable in order to reduce the overall structural volume of the vacuum pump, these difficulties increase because of the resultant increased power density.
Regarding the housing, there exists the possibility of dissipating the heat by means of a water or air cooling system. However, the dissipation of heat from the rotating, rotary pistons is essentially effected only by the conveyed medium itself which either transfers the heat from the rotary piston to the housing or carries the heat along with it. Since operation of twin-shaft pumps in a vacuum makes available only relatively few molecules to carry away the heat, the thermal problems in this field are particularly critical.